I have a 44-77 I have played around with a little bit and from what I have seen, it seems to shoot just fine right from the get go. I have plans to add 2 more BN cases to the heard. a 40-50 bn and a 44-60 SBN. I have watched a friend shoot his 40-50  and see nothing at all wrong with its accuracy. 
From my point of view the only issue or problem I see with BN cases is  a lack of flexibility in load volume. In Straight cases you can lower the powder charge with no problems, just seat deeper to an extent. BN cases are not quite that flexible. Its pretty much full charge or nothing.  Good fouling control is important, but its easily managed in a match  and in a hunting situation you are probably good for 3 shots. By that time it should either be dead or gone. 
One drawback that does really irritate me a lot is what they charge for dies.  The process for making a 3 die set is pretty much the same no matter what the caliber. You buy the reamers and make the dies. Yea I can see a little extra for odd calibers but a 3 die set for 40-65 is $185 and a 3 die set for and a 40-50 bn die set is $265. Step up to the 44-60 and you are up to $305. These are all prices from Buffalo Arms and all CH dies. 
In the past there was a wide variety of BN cases. They had to work to an extent back then. No real reason they will not work today, with in reason.  It just may take a little more work to get they to where you want them to be. 
Sam